*Note - this report may contain independently supported projects which complement the work in this GRDC research program.

Take Home Messages

• There are many virus species and their incidence is variable between sites and seasons.
• Virus problems are increasing. The number of species or else incidence of those species appears to have increased since the 1980s.
• Correct identification and monitoring are important for assessing risks.
• As yet there are few control measures specific to viruses.
• The first step to reducing losses from viruses is to follow "best management" practices including retention of cereal stubble adequate seeding rates recommended sowing times and weed control.

Viruses in chickpea and faba bean

Identification of pulse virus species in NSW and Qld. has been carried out since 1992 a season when there were unprecedented losses in chickpea crops on the Liverpool Plains (see list of collaborators under Acknowledgements). A complete list of species detected to date and their abbreviations is given in Table 1.

In 1992-1995 11 species of virus were tested for and 5 of these accounted for most virus-like symptoms in pulses. Symptoms in chickpea included premature death shoot tip necrosis yellowing or reddening. Faba beans showed all of these symptoms and also mosaic ringspots and leaf distortion. In chickpea four species (SCRLV BLRV CMV and AMV) were involved. In faba bean BLRV and BYMV accounted for most of the symptoms. The five viruses mentioned above also accounted for symptoms in other legumes including field pea lentil narbon bean vetch clovers and medics.

In 1996 an later years however these 5 viruses did not account for symptoms in chickpea and faba bean at some sites. From 1997-2002 a broader range of tests (27 different tests with up to 16 tests on individual specimens) was conducted together with electron microscopy to try to establish whether further species of virus were present. This revealed 14 species infecting chickpea and/or faba bean (Table 1). There are likely to be other species that have not yet been identified. Evidence from a variety of sources suggests that BLRV BWYV and CCDV became prevalent in that order from 1992 to 2002 and were rare or absent before that. BLRV and CCDV were unrecorded previously in Australia. Identification of CCDV needs confirmation additional to the one serological test done so far.

Problems with viruses were most severe on the northwest and central slopes particularly areas around Narrabri and the Liverpool Plains. However within these districts the incidence of virus infection differed widely with some localities unaffected and a low incidence in some seasons.

Yield losses of 90% and 40% have been recorded in chickpea and faba bean respectively based on small plot experiments. On the Liverpool Plains virus-affected crops of both species were occasionally not harvested by growers-effectively 100% loss. Symptoms on individual plants tend to be more severe in chickpea than in faba bean but infection incidences are probably higher in faba bean.

Viruses in canola

Widespread virus-like reddening and stunting of canola were noticed in 1999 and again in 2001 and 2002 particularly in southern NSW. It was not established whether these symptoms were due to virus but they prompted virus testing on a limited number of samples.

Three virus species were identified in canola BWYV CaMV and TuMV (Table 1). These same species have also been reported for surveys of canola crops in Western Australia. In Australia these viruses have not been associated consistently with any symptoms in canola. BWYV may cause transient reddening in young plants and ill-thrift in older plants. Both CaMV and TuMV can cause severe symptoms in other Brassica species. Mustard (Brassica juncea) trials in northern NSW have often shown severe mosaic symptoms while adjacent canola remains symptomless.

BWYV has been reported to reduce yield by up to 46% even in the absence of symptoms based on field experiments in Western Australia (reported by Roger Jones and Jenny Hawkes Crop Updates 2002 Department of Agriculture WA). It is also the most common of the three viruses in WA and high infection incidence has also been shown in NSW. BWYV is therefore likely to be a significant problem in NSW although yield loss estimates to date are based on a worst-case scenario of very early artificial infestation and near 100% infection incidence.

Control options and considerations

Plant viruses are sub-microscopic disease agents. They depend entirely on host plants for reproduction living inside plant cells. Once a plant is infected it cannot be cured.

Viruses are spread by vectors mostly insects. They survive in live plants airborne (or rarely soil borne) vectors and sometimes seed and pollen. Control measures which may be effective and practicable for field crops include reducing entry or spread within crops by vectors (pesticides or cultural practices) elimination of infection sources (quarantine elimination of infected seed or weeds) avoiding infection by means of resistant cultivars or reducing effects of infection by means of tolerant cultivars. Whether a particular control measure is effective depends on the species of virus (in particular whether it is transmitted in persistent or non-persistent manner as per Table 1 or in seed) the species of vector (whether susceptible or resistant to treatments) and the crop species (whether there are problems using the measure on a particular crop and whether the crop is colonised by vectors). Whether a particular control measure is practicable depends on cost vs. potential returns likelihood that disease will occur and whether the measure is compatible with best management practices. Developing an effective and practicable control strategy requires correct identification and monitoring of virus species by specific laboratory tests.

At present practicable control measures are limited to the following

For chickpea faba bean and canola

1. Follow best management recommendations including retention of standing cereal stubble which deters aphids using no less than recommended seeding rates sowing on time and controlling weeds. Poorly-established weedy crops suffer most from viruses.
   
   
2. If virus-like symptoms are observed arrange for virus testing to be done (see contact phone and email below). This can establish which virus species occur in the area and whether viruses account for symptoms.
   
   
3. If it is established early and while infection is at a low incidence that a persistently-transmitted virus (Table 1) is present controlling aphids with a registered aphicide foliar spray may be of benefit for canola or faba beans. Early infestation of canola with green peach aphids (reported to be the main vector of BWYV) may also warrant an aphidicide spray in areas where BWYV is known to occur. This should be followed by examination of sprayed plants to see if the aphid population is resistant.

For chickpea

1. In areas with a history of virus problems consider planting cultivar Gully which has partial resistance to infection with at least three viruses. Gully is particularly susceptible to foliar diseases so careful management with fungicides is required.
   
   
2. Note that in small plot trials seed and foliar insecticides have so far failed to give any reduction of viruses in chickpea.

For canola

1. Gaucho® (imidacloprid) is a registered seed treatment for red legged earth mite and blue oat mite in canola. Recent trials by Dr. Roger Jones' group in Western Australia described in a recent media release by the Centre for Legumes in Mediterranean Agriculture indicate that Gaucho®-treated seed may be advantageous where BWYV occurs at a high incidence.

Virus testing can be carried out free of charge as part of surveys being conducted in DAN00023. For information on collection of samples please contact Mark Schwinghamer.

Table 1. Species of virus identified in chickpea faba bean and canola in NSW or QLD

+ denotes records substantiated by several criteria; * denotes records based on one serological test only
Vectors A aphids; T thrips; L leafhoppers
N non-persistent (persists in vector for a short time not more than 4 hours and usually less)
P persistent (persists in vector for more than 4 days sometimes for the life of the vector)
S semi-persistent (sometimes persists in vector for up to 4 days)

Acknowledgements
Work in this paper was carried out within DAN443 and other projects funded by GRDC and ACIAR from 1992 to present. The authors acknowledge results and information provided by Joop van Leur Kevin Moore John Holland and Rod Bambach (NSW Agriculture Tamworth) Gordon Murray and Kerry Wratten (NSW Agriculture Wagga) John Seidel (Serve-Ag Albury) and Lindrea Latham (Department of Agriculture Western Australia). Virus tests included work by Dr. Gradon Johnstone and Peter Cross (TASAG Hobart) Drs. Khaled Makkouk and Safaa Kumari (ICARDA Syria) Dr. Catherine Johnston-Lord (University of Sydney now AQIS Eastern Creek) and Dr. Angela Freeman (DNRE Horsham) as well as by us at Tamworth and also electron microscopy by Dr. Ray Pares (formerly NSW Agriculture Rydalmere) and Dr. Mukesh Srivastava (NSW Agriculture Camden).